This invention relates generally to controlling the operation of a plant, particularly a cryogenic rectification plant, and is particularly useful for controlling the start up of a cryogenic rectification plant.
The start up of any industrial plant is a complex task whose proper execution requires a significant amount of expertise on the part of the operator(s). Due to the complexity of the procedures, even the most skilled operators can deviate significantly from established xe2x80x9cbest practicesxe2x80x9d, resulting in increased start up times, wasted energy, reduced equipment life, etc. Additionally, in environments where the operation of groups of plants is centralized (i.e. one operator is responsible for the operation of more than one plant), the attention required to start up one plant takes away manpower available to operate others. It is therefore desirable to automate start ups as fully as possible.
The current state of the art when it comes to automating a cryogenic rectification plant during start up involves a xe2x80x9crecipexe2x80x9d, based program. Here, the word xe2x80x9crecipexe2x80x9d is used to mean that events are carried out in the same manner regardless of how the plant is responding to the events. Many plants have programs designed to start up equipment and ramp key flow rates and ratios in a consistent fashion for every automated start (consistent here meaning that equipment is always started in the same order and key flows and flow ratios are always ramped from their starting values to operator specified target values in the same fashion for each start up). While a xe2x80x9crecipexe2x80x9d based program can significantly decrease the manpower required to start up a plant and significantly improve start up reliability, there are some key areas where improvements can be made. In particular, it is often necessary for an operator to closely observe an automated start and take corrective action when the xe2x80x9crecipexe2x80x9d based program performs actions that are inappropriate given the current state of the plant. If the corrective actions performed by the operator are found to be of a repetitive nature (focusing on the same tasks each time) and his/her actions follow a set of well-defined rules (as in the case of liquid inventory management), then these activities can be automated.
The main obstacle to automating the activities of an operator for start up is that the behavior of the plant under start up conditions is non-conducive to traditional feedback control techniques. The non-linear nature of the plant makes it difficult to design a traditional controller that will provide reasonable performance while maintaining stability. The operator, unlike a traditional controller, is able to take the proper actions because his/her extensive experience with the plant provides an intuitive feel for how the plant will respond to certain actions. Such intuition is difficult to incorporate into a traditional controller and is perhaps the reason that xe2x80x9crecipexe2x80x9d based systems (systems that lack feedback) remain popular.
Accordingly it is an object of this invention to provide an improved method for controlling a plant, particularly a cryogenic rectification plant, which can be effectively used in the start up of the plant.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for controlling the operation of a plant comprising:
(A) determining the value of a plant process parameter, comparing the determined value to the desired value for that parameter, calculating the difference between the determined value and the desired value, and calculating the rate of change of said difference between the determined value and the desired value;
(B) qualitatively categorizing the said difference between the determined value and the desired value, and qualitatively categorizing the said rate of change of said difference between the determined value and the desired value;
(C) determining by referring to a set of rules for qualitative categories how such process parameter should be changed to become closer to the desired value;
(D) determining a numerical value for adjusting a plant element to change such process parameter to become closer to the desired value; and
(E) adjusting said plant element to change such process parameter so that it is closer to the desired value.